There’s a huge amount of variation in recipes, but they all have two things in common: tomatoes (naturally!) and thin-sliced raw onion that’s been tempered in some way – there’s recipes that call for a soak in cold water, a quick blanch, or to marinate it in red wine vinegar. I went with the the last one. I used this recipe from Food&Wine and halved it for a single portion.
The onion still has lots of crunch but the flavour is much milder and blended into the red wine vinegar – they take each others’ sharpness away. It’s a nice taste, I might try to crush in some garlic or add some lemon juice next time, play with the variations a bit. The marinated onions would also be a great addition to a bocconcini salad.